Sports participation in the United States and the world has significantly increased over the past half decade. Recent U.S. Census data reflects these trends in athletics. According to census reports, in 1971 approximately 3.9 million high school students were involved in organized athletics. This compares to the 1998–1999 school year when approximately 6.5 million students participated in sports. These trends are also seen in organized collegiate sports participation, to say nothing of the dramatic increase within the general population of the number of people involved in recreational athletic activities.
In addition to the rise in athletic participation over the past thirty years, there has been a significant change in the demographics regarding who is involved in athletic activities. The implementation of Title IX legislation is largely responsible for this changing demographic. In particular, Title IX has been widely credited with the dramatic rise in female sports participation. With the increase in sports participation has come an increase in sports-related injuries.
Participation in sports, by definition, involves a risk of injury. The goal of sports medicine specialists is to define the injury patterns and mechanisms by which these injuries occur. As a greater fund of knowledge has accumulated regarding sports-related injuries, the aim of researchers and clinicians has shifted toward identifying what risk factors can be controlled in an effort to prevent injury.
Serious knee injuries, such as anterior cruciate ligament (ACL) tears have become commonplace at all levels of participation ranging from high school sports to professional athletes. It has been estimated that 80,000 ACL injuries occur annually in the United States. The vast majority of these injuries are sports related. Injury rates greater than 1 in every 3000 sports participants have been reported. An estimated $1 billion is spent annually to treat these injuries.
With the rapid rise of female athletics, has come very disturbing trends regarding the injury rates between male and female athletes. Studies have reported ACL injuries are 2 to 8 times more common in female athletes when compared to their male counterparts. In addition, approximately 70% of the catastrophic knee ligament injuries occurring in sports are non-contact in nature.
In order to understand the rationale behind preventing ACL injuries, it is first necessary to have a basic knowledge of the anatomy of the knee joint. The knee is typically described as a hinge-type joint. In reality the knee does not behave as a simple hinge. It has six degrees of freedom, including translation and rotational motions. As with all joints, the stability of the knee is determined by the complex interplay between the bony architecture, the static stabilizers and the dynamic stabilizer (i.e., muscles) around the joint.
The anatomy of the knee joint includes three bones—the distal femur (aka, thighbone), the proximal tibia (aka, shinbone) and the patella (aka, kneecap). The static stabilizers of the knee include four strong ligaments. They are responsible for tightly binding the femur and tibia together. These ligaments not only provide the static stability to the joint, but also the direction in which they run determines the plane of motion that the joint can move in. The four major knee ligaments are the following: (1) medial collateral ligament, (2) lateral collateral ligament, (3) posterior cruciate ligament and (4) anterior cruciate ligament.
The ACL attaches to both the femur and tibia. It is the primary restraint preventing anterior tibial translation (i.e., it prevents the tibia from sliding forward on the femur). It is a secondary restraint to internal rotation, varus-valgus angulation and knee hyperextension. In essence it allows athletes to perform decelerating, twisting and pivoting activities. When the ACL is torn athletes are left with a“trick” knee that may give out during such activities.
It is a well-recognized fact that the risk of injury is inherent in any athletic activity. While it was once a widely held belief that most knee ligament injuries in sports were the result of direct contact between players, research has convincingly shown that this is not the case. The majority of ACL injuries occurring in sports are not related to contact between participants. They are in fact non-contact injuries.
Much research has been directed at identifying as precisely as possible the risk factors for ACL injury in order to determine which risk factors are preventable. Risk factors associated with catastrophic knee injuries can be categorized into four main areas. These categories include the following: (1) environmental risks, (2) anatomic risks, (3) hormonal risks and (4) biomechanical risks. While there may be opportunity for risk reduction in all of these areas, simple changes in environmental factors may have profound influence over the risk of ACL injuries. Environmental risk factors associated with knee injury encompass such areas as equipment, braces, and the interaction between the playing surface and the athletic shoe.
Since the mid-1970's it has been known that there is a direct correlation between knee injuries and friction between the athlete and the athletic surface. A study examining the incidence of knee injuries in football players showed that as the friction between the playing shoe and the playing surface increased there was an increase in knee injuries. This finding was not unique to football injuries, it has been shown with injuries in tennis as well. A study in 2002 of ACL injuries in military recruits during obstacle coarse training also proved the role of increased friction at the sole-surface interface as being directly related to the risk of ACL injuries.
At its most basic level, the ultimate mechanism that causes a ligament injury is that the load applied to the ligament is larger than the ligament's capacity to sustain it. Review of videotape of athletes suffering actual non-contact ACL tears reveals that most frequently these injuries occur when the athlete is decelerating, changing direction or landing from a jump. Analysis further shows that the player usually has landed “awkwardly” with their foot flat and the center of gravity of the body behind the center of the knee.
Laboratory data combined with videotape of athletes suffering tears of their ACLs explain why these injuries occur during cutting, decelerating and landing activities. It has been shown that when landing from a jump the angle of the athletes knee at impact is on average 22 degrees. Studies have documented that the anterior shear force across the knee created by a quadriceps contraction is the greatest when the knee is flexed between 0–30 degrees. Furthermore, the muscle firing data confirms that as an athlete lands from a jump the quadriceps muscle is maximally firing which places a maximal anterior shear force across the knee. Add to this the fact that at the same time the quadriceps is drawing the tibia forward the hamstrings are only minimally active to counter activate this strong forward pull. This combination of marked quadriceps activity and minimal hamstring activity places the ACL under significant tension and increases the risk of ligament rupture.
It has also been documented in gait lab analysis that the body's corrective action in an attempt to regain balance and control when landing “awkwardly” is to maximally contract the quadriceps muscle. The reason that a strong quadriceps contraction can provide an anterior shear force on the ACL of sufficient magnitude to rupture the ligament is that the foot is fixed to the playing surface. If the foot is not fixed to the playing surface, then when the quadriceps contracts the muscle simply extends the knee and no anterior shear force is created in the knee. Therefore, by controlling the shoe-surface interface, we can eliminate the anterior shear and in doing so we can help prevent non-contact ACL injuries.
Various break-away shoes have been designed in efforts to reduce the incidence of injuries in athletes. For example, U.S. Pat. No. 3,668,792 to York describes an athletic safety shoe having an upper sole mounted on the body of a shoe and a lower breakaway safety sole having traction means on the underside thereof which is releasably attached to the upper sole by a breakaway safety mechanism. The safety sole has a generally transversely extending grooved track configured to slideably receive a generally transverse rib. The breakaway sole is designed to prevent longitudinal (anterior-posterior) movement between the upper and lower soles and to allow transverse (medial-lateral) movement between the upper and lower soles when a threshold transverse shearing force is applied therebetween.
U.S. Pat. No. 5,617,653 to Walker et al. (hereinafter “Walker et al.”) describes a cleat assembly for athletic shoes which includes a base assembly and a cleat which is releasably coupled to the base assembly. The cleat is designed to release from the base assembly in response to a predetermined force extending substantially lateral to the longitudinal axis of the shoe. The cleat described in Walker et al. allows for a release in a transverse (medial-lateral) direction.
U.S. Pat. No. 5,867,923 to Lehneis discloses an orthotic shoe 10, having an insole 14 and an outsole 16 that are mounted to a pivot 18 to allow relative rotation therebetween about an axis perpendicular to the sole.
U.S. Pat. No. 5,224,810 to Pitkin discloses an athletic shoe designed to provide a safe orientation of the foot during an immediate stop in the medial lateral direction. The shoe sole has an upper sole member 3 and a lower sole member 4 which are elastically connected by a resilient member 5 along the lateral and medial edges of the shoe. The elastically connected sole members allow motion therebetween during rapid stopping in the medial direction.
U.S. Pat. No. 3,982,336 to Herrog discloses an athletic shoe with a breakaway sole. The shoe has grooves 26, 28 and 30 and projections 32, 34 and 36 to allow the lower sole 16 to be released from the upper sole 14 when a lateral force is applied across the shoe 12 which would be incurred when an injurious or harmful force is applied to the leg of an athlete.
These and other previously known safety shoe designs were neither designed nor intended to release in a longitudinal direction. As such, there is a need for a shoe sole design that helps to prevent ACL injuries by ensuring that the anterior shear force at the knee does not exceed the tensile strength of the ligament.